Typical photodetectors are composed of semiconductors that convert incident light into either a current or voltage, depending on how the photodetectors are configured to operate. Consider for example a simple pn or p-i-n junction photodiode. When a beam of photons with energies exceeding the band gap of the photodiode strike the photodiode, most of the photons are absorbed by exciting electrons from the valence band into the conduction band, thereby creating free electrons in the conduction band and a corresponding number of free, positively-charged holes in the valence band. When the absorption occurs in the junction's depletion region, or at least one diffusion length away from the depletion region, electrons in the conduction band and holes in the valence band are swept from the junction by an electric field formed across the depletion region. The field drives holes toward an anode and electrons toward a cathode to produce a photocurrent. In this same manner, typical semiconductor-based photodetectors destroy much of the incident light via absorption.
In recent years, radiation pressure-based photodetectors have been developed to detect light without destroying the light. Radiation pressure is the pressure exerted by an incident beam of light upon a surface located in the path of the beam. However, detecting light with a radiation pressure-based photodetector involves transferring the linear momentum of the beam to a mechanical object, such as a mirror connected to a pressure sensor, which in turn changes the direction of the beam. Physicists and engineers seek systems for detecting a beam of light without destroying the light or changing the direction of the beam.